The present invention relates to microwave assisted chemical processes and the equipment used to carry them out. Microwaves can heat various materials, including chemical reactants, by interacting directly with the materials to cause molecular motion and generate heat. As a result, the effect of microwaves on certain materials, including water and a number of organic liquids and related materials, is very rapid compared to more conventional heating techniques. This rapid reaction between microwaves and such materials offers particular advantages in heating certain reactions which otherwise would have to be heated by conduction or convection processes. Because convection and conduction transfer heat between adjacent objects (for example, a beaker on a hot plate), they can be relatively slow and thus disadvantageous in certain circumstances.
In particular, where ongoing processes such as commercial manufacturing requires chemical testing and analysis that in turn may require heating steps, the lag time required by conventional heating techniques can result in a similar lag time in the production process itself. Thus, to the extent that these tests (or sample preparation for such tests) can be done more quickly, the resulting processes can be carried out more quickly as well. Accordingly, the use of microwave techniques has greatly increased the speed and frequency at which certain types of chemical analysis can take place.
Accordingly, microwave assisted techniques have been developed for drying, moisture analysis, digestion, extraction, and other related techniques.
There are a number of ways to carry out microwave assisted chemical reactions. In some techniques, the reaction is most advantageously carried out in a sealed, pressure-resistant vessel in which the reaction can proceed under conditions of both elevated temperature and elevated pressure. In other circumstances, microwave assisted chemical reactions are carried out under "open" conditions; i.e., at ambient pressure. Some of these open systems use reaction vessels that resemble (as a somewhat simplistic description) test tubes; i.e., cylindrical vessels with one closed end and one open end. It will be understood that as used herein, "open" refers to systems that are in general equilibrium with the ambient atmosphere. Thus, an "open" vessel can actually have a lid, but it will also have some sort of a vent that allows it to come to equilibrium with ambient pressure.
The test tube shape of such vessels is often helpful in certain techniques in which the lower end of the vessel is supported in a cavity into which microwaves are sent from their source, typically a magnetron, and through a waveguide to the cavity. The vessel extends into the cavity through a microwave attenuating device that is commonly referred to a "choke." A choke is a structural feature, sometimes as simple as a short piece of tubing that, although otherwise open to its surroundings, will block the passage of microwaves therefrom. Accordingly, in a typical vertical arrangement, a reaction vessel will be maintained vertically with its lower portion holding a sample in the microwave cavity, and with its upper portions supported by the choke, and with some vent or other opening at the top to allow it to come to equilibrium with the surrounding atmosphere.
A number of commercially available microwave devices use such a vertical orientation of open vessels, the most recent of which is the STAR system available from CEM Corporation of Matthews, N.C. and which is thoroughly described in commonly assigned application Ser. No. 08/538,745, filed Oct. 3, 1995, and now U.S. Pat. No. 5,796,080. The disclosure of the '745 patent application is incorporated entirely herein by reference.
The vertical orientation of such vessels, however, has led to a somewhat unexpected problem in some circumstances; mainly, liquids that are heated in the lower portions of the vessel, and then evaporate must travel upwardly through and out of the remainder of the vessel. Because only a portion of the vessel is typically maintained in the cavity, however, these vapors can tend to cool while they are within the upper portions of the vessel, and in some circumstances condense on the inside walls of the vessel. When such occurs, the condensate returns by the action of gravity to the remainder of the liquid sample; i.e., an unintentional refluxing.
In many circumstances such unintentional refluxing makes little or no difference in the overall chemical process being carried out. There are other processes, however, for which microwave-assisted heating is quite desirable but for which the refluxing effect is disadvantageous. For example, microwave assisted chemistry is now being used as one of the sample preparation steps in acid purity testing for the semiconductor industry. As those familiar with that industry are aware, the materials used in certain steps must be of extremely high purity; i.e., with impurity levels at the part per trillion level or less. In order to test the purity of the acid, while avoiding the harsh effects of concentrated acid on the testing equipment, a larger sample of acid (e.g., 50 milliliters) is typically evaporated to a much smaller volume (e.g., 1-2 ml) then diluted with highly purified water and analyzed. In testing the resulting diluted sample, the presence of acid can be quite disadvantageous because it changes the viscosity of the solution which in turn effects certain steps in the test, particularly certain aerosol techniques.
It has been found, however, that when using microwave assisted techniques in open vessels of the type described, the refluxing effect can be significant as the vapors from the acid being evaporated recondense near the top of the vessel and run back down to the sample portion.
This is, of course, just one example of the potential disadvantages of the refluxing effect in such vertically oriented open vessels. It nevertheless illustrates the need for reaction vessels for open microwave assisted chemical processes that either minimize or eliminate the refluxing problem in these vessels.